The present invention relates to polysaccharide derivatives containing aldehyde groups on an aromatic ring and to the acetal derivatives used in the preparation of some of the aldehyde-containing polysaccharide derivatives. It also relates to a non-oxidative process for introducing aldehyde groups into polysaccharides, particularly granular polysaccharides. It further relates to the use of the cationic polysaccharide derivatives as paper additives and the use of the starch derivatives in corrugating adhesives.
As used herein, the term "paper" includes sheet-like masses and molded products made from fibrous cellulosic materials which may be derived from natural sources as well as from synthetics such as polyamides, polyesters, and polyacrylic resins, and from material fibers such as asbestos and glass. In addition, papers, made from combinations of cellulosic and synthetic materials are applicable herein. Paper-board is also included within the broad term "paper", with "corrugated paperboard" referring to a fluted medium and a facing adhesively joined to the tips on one or both sides of the fluted medium.
Oxidative and non-oxidative methods have been used to introduce aldehyde groups into polysaccharides such as starches, gums, and celluloses. The oxidative methods used have included treatment with periodic acid, periodates, or alkali metal ferrates. See U.S. Pat. No. 3,086,969 (issued Apr. 23, 1963 to J. E. Slager) which discloses an improved process for the preparation of a dialdehyde polysaccharide, (e.g., starch) using periodic acid; U.S. Pat. No. 3,062,652 (issued Nov. 6, 1962 to R. A. Jeffreys et al.) which discloses the preparation of dialdehyde gums (e.g., gum acacia, pectin, and guar) using periodate or periodic acid; and U.S. Pat. No. 3,632,802 (issued Jan. 4, 1972 to J. N. BeMiller et al.) which discloses a method for oxidizing a carbohydrate, (e.g., starch or cellulose) with an alkali metal ferrate.
In the above methods the aldehyde groups are formed by the oxidation of the hydroxyl groups on the ring and/or side chain. Treatment with periodic acid or periodate selectively oxidizes the adjacent secondary hydroxyl groups on the ring carbon atoms (e.g., the 2,3-glycol structures), cleaves the ring, and results in a "so-called" dialdehyde derivative which is principally a hydrated hemialdal and intra- and intermolecular hemiacetals. Treatment of carbohydrates with alkali metal ferrates selectively oxidizes the primary alcohol group on the side chains without ring cleavage or oxidation of the ring hydroxyls.
The disadvantages of the oxidative method include degradation to lower molecular weight products and the formation of carboxyl groups due to further oxidation of the aldehyde groups. U.S. Pat. No. 3,553,193 (issued Jan. 5, 1973 to D. H. LeRoy et al.) describes a method for oxidizing starch using an alkali metal bromite or hypobromite under carefully controlled conditions. The resulting dialdehyde is reported to have a substantially greater proportion of carbonyl groups (i.e., aldehyde groups) than carboxyl groups. It also discloses a method for selectively oxidizing the side chains of starch derivatives (e.g., an alkoxylated starch such as dihydroxypropyl starch) under the same process conditions whereby the underivatized starch hydroxy groups on the rings are substantially non-oxidized.
The presence of carboxylic groups in aldehyde starches has several disadvantages in addition to the obvious reduction in the degree of aldehyde substitution. These include the introduction of hydrophilic properties due to the carboxyl groups, an upset in the cationic/anionic ratio when a cationic starch base is used (as in most papermaking wet end uses), and the possible formation of salts (in the above papermaking end use) which could give rise to ionic crosslinking.
The non-oxidative methods typically involve the reaction of the polysaccharide with an aldehyde-containing reagent. See U.S. Pat. No. 3,519,618 (issued July 7, 1970 to S. M. Parmerter) and U.S. Pat. No. 3,740,391 (issued June 19, 1973 to L. L. Williams et al.) which cover starch derivatives and U.S. Pat. No. 2,803,558 (issued Aug. 20, 1957 to G. D. Fronmuller) which covers a gum derivative. The starch derivative of Parmerter is prepared by reaction with an unsaturated aldehyde (e.g., acrolein) and has the structure Starch-O--CH(R.sup.1)--CH(R.sup.2)--CHO where R.sup.1 and R.sup.2 are hydrogen, lower alkyls or halogen. The starch derivative of Williams is prepared by reaction with acrylamide followed by reaction with glyoxal and has the structure ##STR3## The gum derivative of Fronmuller is prepared by treating the dry gum (e.g., locust bean or guar gum) with peracetic acid to reduce the viscosity, neutralizing, and then reacting with glyoxal. Water-soluble cellulose ethers (e.g., hydroxyethylcellulose) have also been reacted with glyoxal or ureaformaldehyde to give aldehyde-containing derivatives.
One of the disadvantages of introducing the aldehyde groups directly using an aldehyde-containing reagent is the possibility of the derivative crosslinking prior to use. The Williams patent (cited above) alludes to this problem when it notes that solutions of the glyoxalated polymers "are stable for at least a week when diluted to 10% solids by weight and adjusted to pH 3" (see Col. 3, lines 60-63). The Parmerter patent notes that the starch aldehyde is "a substantially non-crosslinked granular starch derivative" and discusses the importance of the non-crosslinked character (see Col. 2, lines 40-45).
The crosslinking of a starch derivative containing two or more aldehyde groups (e.g., dialdehyde starch) with compounds containing two or more isocyanate groups (e.g., polymethylenepolyphenylene polyisocyanate) provides a water-resistant adhesive composition. See Japan. Kokai 57-202,362 published Dec. 11, 1982, Hohnen Oil Co., Ltd. (C.A. 98: 200152m 1983). The crosslinking of dialdehyde starch with an alkylene diamine such as an aliphatic diamine (e.g., hexamethylene diamine), and/or an aromatic alkylene diamine (e.g., methylene dianiline) is disclosed in U.S. Pat. No. 3,706,633 issued Dec. 19, 1972 to E. Katchalski et al. However, the crosslinking cannot be controlled because the granular dialdehyde starch is reactive toward the starch hydroxyls, thus leading to premature self-crosslinking rather than crosslinking via the diisocyanate or diamine.
Polysaccharides modified with acetals, such as dimethoxyethyl methyl chloroacetamide (DMCA), as described in Ser. No. 758,634 (cited previously), have been shown to possess some very unique properties. Once the polysaccharide is dispersed and the acetal is converted to aldehyde by lowering the pH to less than 7, crosslinking can occur between the aldehyde and any available hydroxyl group. This system is extremely effective as a strength additive in paper where the aldehydes can crosslink the fibers through the cellulose hydroxyls. The main drawback of these aldehydes is their tendency to crosslink the supporting polysaccharide backbone, as evidenced by a large increase in viscosity of a dispersion at greater than 5% non-converted starch solids. This makes their use in coating and adhesive applications, where high solids are required, impractical.
There is therefore a need for an aldehyde system that is non-reactive towards the hydroxyls on the polysaccharide backbone and which will crosslink when desired.
The particular adhesive employed in the corrugating process is selected on the basis of several factors, including the type of bond required in the final application of the finished corrugated product. Starch-based adhesives are most commonly used due to their desirable adhesive properties, low cost and ease of preparation.
It is often desired or necessary in the manufacture of corrugated paperboard that the adhesive yield water-resistant bonds which can withstand extended exposure to high humidity, liquid water, melting ice and the like. A number of approaches have been devised to produce water-resistant corrugating adhesives. One method involves the preparation of an acidic, starch-based adhesive wherein urea-formaldehyde resin is added to the composition, together with an acidic catalyst such as aluminum sulfate, to produce water-resistant bonds in the corrugated board manufactured therewith. The adhesive composition itself, however, is deficient in other important properties such as corrugator bonding speeds, viscosity stability, and pot life and exhibits excessive formaldehyde odor. In addition, acidic corrugating adhesives tend to be corrosive.
The many disadvantages associated with the acidic corrugating adhesives led to the development of water-resistant alkaline curing starch-based adhesives for use in the corrugating industry. In the preparation thereof, a thermosetting resin, such as, e.g., ureaformaldehyde, resorcinol-formaldehyde, melamine-formaldehyde, phenolformaldehyde, acetone-formaldehyde, ketone-aldehyde and urea-acetone-formaldehyde condensate, has been added to the adhesive as a cross-linking additive for the amylaceous components to produce water-resistant bonds.
In using thermoset resins of the type mentioned above, crosslinking occurs immediately upon addition of the resin. This causes thickening of the cooked portion and inhibition of the uncooked portion, both of which result in poor speeds on the corrugator after the adhesives age for a number of hours.
The corrugating industry is still searching for means for providing water resistance to corrugated paperboard products prepared from alkaline curing starch-based adhesives which are formaldehyde-free and which do not crosslink immediately.